1. In General
This invention relates to antennas and, in particular, to four beam printed antennas for Doppler applications. Accordingly, it is a general purpose of this invention to provide novel devices of such character.
For a fixed antenna Doppler velocity measuring radar system, the antenna must sequentially generate four independent but symmetrically located pencil beams, each in a different quadrant of the ground. It is also necessary in Doppler radar applications to generate low side lobe, narrow beamwidth patterns. The latter two requirements generally require that all four beams be radiated from a common aperture in order to more fully utilize the space allocated for the antenna.
The space or volume allocated for the antenna is another very important consideration. In the past, planar antennas were required to be imbedded inside the frame of the aircraft and a conformal radome used to enclose the antenna. The use of a conformal or cylindrical antenna would be most desirable since the overall volume occupied by the Doppler radar antenna can be substantially reduced.
This invention provides a novel approach for generating four symmetrically located pencil beams with low side lobe levels from a common cylindrically shaped photo etched antenna. The approach for generating the four beams, one in each of the four spatial quadrants, is accomplished without the use of active phase shifting devices and without the use of amplitude controllers. It is believed that this has never been previously accomplished in cylindrical arrays without the use of active phase shifters and amplitude controllers.
Another novel feature for this invention is the thickness of the antenna. This antenna, constructed in accordance with the invention, is very thin and thus can be mounted on the outside of an aircraft. A thick antenna cannot be mounted on the outside surface since severe aerodynamic perturbations would result. However, if the antenna is to be imbedded inside the airframe, a cut-out is required. Any cut-out in the airframe is undesirable since the structural integrity of the frame is compromised. Even if structurally the cut-out is acceptable, it would still be desirable to mount the antenna on the surface for economic considerations.
This invention requires only that a few small holes be made in the airframe. The holes are only required to connect the antenna to the electronic circuitry located inside the aircraft. An illustrative mounting arrangement is shown in FIGS. 19a, 19b, 19c, 19d, and 19e where a cylindrical antenna is shown mounted outside the aircraft and conforming to the aircraft shape.
The concept presented can similarly be used to generate four symmetrically located pencil beams with low side lobes from a planar photo etched antenna. Because the description of the antenna is most efficiently approached by describing the planar configuration, a description of the planar case will procede the description of the cylindrical case.
2. Description of the Prior Art
It is desired to provide novel antenna configurations that can be used for low cost printed, or photo-etched, planar or conformal Doppler applications. The state of the art of antennas has advanced to a degree that printed antennas can be provided in extremely thin shape so that they can be made in conformal shapes to meet the requirements for difficult aircraft installation problems. A printed antenna, such as described in U.S. Pat. No. 3,721,988, is not suitable for four beam conformal applications.
Multi-beam antennas have been successfully built, for example, as set forth in "An RF Multiple Beam-Forming Technique" by William P. Delaney, IRE Transcations on Military Electronics, April, 1962, pages 179 to 186. Delaney describes a sixteen beam antenna.
In order to achieve a low side lobe performance desired for fixed antenna Doppler radar, it is desired that a tapered rather than a uniform illumination be generated by the matrix. It has been shown by R. C. Hansen in "Microwave Scanning Antennas", Volume 111, at pages 263 to 268, that by connecting the beam ports of two adjacent beams of a uniformly illuminated matrix, the resulting amplitude function is a cosine with a tapered side lobe structure with the first side lobe-23 db down from the main beam.
This invention incorporates elements known as Butler matrices. The design on the feed matrix is described in "The Systematic Design of the Butler Matrix" by H. J. Moody, IEEE Transactions on Antennas and Propagations" November 1964, pages 786 to 788.
This invention further utilizes strip radiators. The strip radiators can include devices known as Microstrip or strip line. Many of these applicable radiators are described in the literature, for example, "The Sandwich Wire Antenna" by W. Rotman and N. Karas, The Micro-Wave Journal, August 1959. Also, see "Recent Developments in the Study of Printed Antennas" by J. A. McDonough, R. G. Malich, and J. Kowalsky, AIL, 1957. Another suitable radiator would be the so-called Collins radiator, described in "A New Flush Mounted Antenna Element for Phased Array Application" By E. V. Byron, Proceedings of the Phased Array Antenna Symposium, 1970.
Still another suitable radiator, would be a simple short section of a conductive strip located above a ground plane. Such short open circuit stub radiators are described by L. Lewin in "Radiation From Discontinuities in Strip-line", Proceedings of the IEE (England), 1960.
Additional references on Butler matrices include "Multiple Beams from Linear Arrays," J. P. Shelton, IRE Transactions on Antennas and Propagation, March 1961. See also, "The Design of Hybrid of Multiple Beam Forming Networks" by K. H. Hiring, Proceedings of the 1970 Phased Array Antenna Symposium.
Various references have described basic concepts for scanning a circular array in detail. Such references include "A Matrix-Fed Circular Array for Continuous Scanning" by Boris Sheleg, Proceedings of the IEEE, Volume 56, No. 11, pages 2,016 to 2,127, November 1968; "The Multiple-Beam Cylindrical Array Antenna-Theory" by George Chadwich, Conformal Array Antenna Conference, NELC, San Diego, California, January 13-15, 1970; and "The Multiple-Beam Cylindrical Array Antenna-Practice" by R. Van Wagoner, Conformal Array Antenna Conference, NELC, 1970.
Though the basic concepts for scanning a circular array is discussed in the foregoing three references a less vigorous discussion is given below. The far field radiation pattern for the linear array is given by ##EQU1## where ##EQU2## The far field radiation pattern for a circular array is given by ##EQU3## Note that in Equation (1) above, there is a linear phase variation across the element function A.sub.n. However, because C.sub.n in equation (2) is a complex function there is no one-to-one correspondence between changes in the current distribution and C.sub.n. If the current distribution about the ring were to be represented by a Fourier series, the terms of such series would each represent a current mode of unit amplitude having a linear phase variation with angle. The far field pattern for each mode has the same form as the mode itself. Thus, it is proper to consider these mode patterns to be terms in the Fourier representation of the far field pattern of the original distribution. It then becomes apparent, that by expressing the far field pattern to the sum of the modes, each having a linear phase variation, the analogy to that of a pattern for a linear array is relevant.
Shelton discovered that it is possible to excite these modes simultaneously and independently by using a Butler matrix to feed N elements. See "Multiple Beams from Linear Arrays" by J.P. Shelton, IRE Transactions on Antennas and Propagation, March 1961, pages 154 to 161.